1. Field of the Invention
The present invention is generally related to a graft involving a blood vessel segment and a supportive sheath chosen to provide the graft with mechanical compliance properties which resemble those of a healthy native artery, and a method for sizing such a graft.
2. Description of Related Art
Various types of vascular prostheses are known or available. Commercially available synthetic vascular grafts in use are commonly made from expanded polytetrafluoroethylene (e-PTFE), or woven, knitted, or velour design polyethylene terephthalate (PET) or Dacron®. These prosthetic vascular grafts may have various drawbacks. When used for repairing or replacing smaller diameter arteries, these grafts may fail due to occlusion by thrombosis or kinking, or due to an anastomotic or neointimal hyperplasia (exuberant cell growth at the interface between artery and graft). Another problem may involve expansion and contraction mismatches between the host artery and the synthetic vascular prosthesis, which may result in anastomotic rupture, stimulated exuberant cell responses, and disturbed flow patterns and increased stresses leading to graft failure.
Problems also exist with the use of autologous saphenous vein grafts in these applications. Use of autologous saphenous vein grafts to bypass blockages in coronary arteries has become a well-established procedure. However, their success in the long term has been limited. In the coronary position, the literature reports a low (45-63%) patency of vein grafts after 10-12 years. It is believed that these failures result from remodeling of the implanted vein in response to greatly increased internal pressure, that is, as the vein is required to function as an artery. In general, arteries have substantial musculature and, although able to expand diametrically in response to increased internal pressure, are capable of withstanding normal arterial pressure variances. Veins, on the other hand, are not required to withstand arterial pressure variances and are relatively incapable of withstanding the higher arterial pressures without substantial bulging. In this regard, the nominal venous diameter seen under nominal venous pressure is seen to approximately double upon exposure to arterial pressure.
Increases in lumenal diameter of these magnitudes in vein segment implants are accompanied by increases in tangential stress. Tangential stress has been shown to be proportional to the lumenal radius-wall thickness ratio. In healthy arteries, this ratio remains constant across multiple species. However, this does not occur in veins. It is believed that a vein's smooth muscle cells increase their growth rate and secrete extra-cellular matrix components in response to such increases in tangential stress. This becomes a remodeling response, and is likely an attempt by the vein to reduce the lumenal radius-wall thickness ratio, and consequently the tangential stress. However, it appears that these reactions overcompensate in the veins, resulting in the phenomenon of neointimal hyperplasia yielding grossly thickened and stiff graft walls. As the dilation of the vein segment continues, the resulting mismatch between the vein and artery diameters may lead to disturbance of flow patterns, which may also favor the formation of thrombi.
Problems also exist when tubular prostheses are used as exteriorly accessible shunts to facilitate access to the circulatory system for, e.g., the administration of medicines and nourishment and for dialysis procedures.
For several decades saphenous vein grafts have been the most widely used arterial bypass conduits. As much as there is an increasing trend towards the use of arterial grafts such as the internal thoracic-, radial- or gastroepiploic artery, the saphenous vein will remain an indispensable conduit for large numbers of patients. This is particularly true for lower limb reconstructions where artery grafts are not available.
Although the overall patency of saphenous vein grafts is distinctly better than that of synthetic conduits, the failure rate of vein grafts is still sobering when compared with artery grafts. The main reason for the failure of vein grafts is the development of intimal hyperplasia. Since late vein graft failure due to arteriosclerotic degeneration also develops on the bed of intimal hyperplasia, this subintimal tissue development holds the master-key to poor vein graft performance. The consequences of this shortcoming are dramatically illustrated by the fact that one third of all peripheral vascular operations are revisions and at 5 years 50% of all peripheral grafts needing revision for failure led to an amputation.
It is well recognized that there are two major forms of intimal hyperplasia: a diffuse and a focal one. While diffuse intimal hyperplasia often regresses, focal intimal hyperplasia tends to progress, leading to a significantly higher occlusion rate. The overall triggers for both forms of intimal hyperplasia are low shear stress at the blood interface and high circumferential wall stress—both related to the significantly larger cross sectional area of the vein graft than the target artery and exposure to arterial pressure. The aggravating factors in focal narrowings, however, are areas of particularly low fluid shear stress and increased shear gradients. Eddy flow as a consequence of uneven lumenal dimensions was shown to be the reason behind these haemodynamic conditions causing focal intima hyperplasia. Independently, wall irregularities were shown to be the main predisposing condition for focal intimal hyperplasia.
As early as in the 1960s attempts were made to restrict the expansion of vein grafts in the arterial circulation and eliminate uneven lumenal dimensions through external mesh-support with diameter reduction. Since then, many investigators have researched this field but the translation into clinical practice was limited to last-resort measures in varicose veins.